Major component of all biological membranes. Essential structural component of mammalian cell membranes and required to establish proper membrane permeability and fluidity. In addition to its importance within cells, also serves as a precursor for the biosynthesis of steroid hormones, bile acids, and vitamin D. Regulates the function of the transporters and signaling proteins present on the plasma membrane. Implicated in cell signaling processes, assisting in the formation of lipid rafts in the plasma membrane, which brings receptor proteins in close proximity with high concentrations of second messenger molecules. Used to prepare immune-stimulating complexes to enhance cellular and humoral responses and suitable for use in liposome preparations. Binds to and affects the gating of a number of ion channels such as the nicotinic acetylcholine receptor, GABAA receptor, and the inward-rectifier potassium ion channel. Also activates the estrogen-related receptor alpha, and may be the endogenous ligand for the receptor. Has been shown to be an important factor in cancer, obesity, neurodeneration and inflammatory diseases.